18 NORTH AMERICAN FAUNA 75 



Marmosa alstoni 

 Alston's Opossum, Zorra or Zorrici 



Alston's opossum, Marmosa alstoni, is found in the Caribbean lowlands and 

 adjacent mountain slopes from Belize to southeastern Costa Rica and in Colombia 

 (Fig. 7). Marmosa alstoni is considered by some to be a subspecies of the widely 

 distributed South American species, M. cinerea. In Costa Rica, Alston's opossum 

 ranges from sea level to about 1,600 m. Although little has been published about 

 the biology or distribution of this species, it is relatively common in appropriate 

 habitats. Vaughan had a group of M. alstoni living in his house near Concepcion 

 at 1,500 m, which is just outside of Parque Nacional Braulio Carrillo. There are 

 no records of this species from La Selva or Braulio Carrillo, but we suspect that 

 it occurs in the park. 



Specimen examined (1)— 1 km NW of Concepcion de San Rafael, 1,500 m 

 [1 sex?, UNA]. 



Marmosa mexicana 

 Mexican Mouse-opossum, Zorra or Zorrici 



Mexican mouse-opossums are found from eastern and southern Mexico to 

 western Panama (Fig. 8). The subspecies Marmosa mexicana zeledoni is found 

 throughout Costa Rica. 



Marmosa mexicana has been seen or taken at La Selva on several occasions, 

 although there are no previously published records of this diminutive opossum from 

 the region. The first specimen from La Selva was a male collected by Casebeer 

 and Starrett in the early 1960's (LACM 24121). The second specimen is a subadult 

 female collected by LaVal on 5 February 1974 (KU 134859). CoUett captured a 

 M. mexicana between the 3- and 4-year-old successional plots at La Selva in sum- 

 mer 1975, and Anderson caught another in a live trap baited with banana there 

 in July 1983; both were released. We have found skulls and jaws of M. mexicana 

 in regurgitated pellets of the spectacled owl (Pulsatrix perspicillata) at La Selva 

 on several occasions. No other evidence of small mammals was found in these 

 pellets, suggesting that mouse opossums compose a significant portion of the diet 

 of spectacled owls there, at least during the rainy season (when the pellets were 

 found). Although there are few records of Mexican mouse-opossums from La Selva, 

 it is likely that they are more common there than the records indicate. They are 

 nocturnal, arboreal, secretive, and seldom observed. Systematic trapping and study 

 of remains found in owl pellets are a much better indicator of their presence and 

 density than are sight observations. 



We collected juvenile, subadult, and adult Mexican mouse-opossums in late 

 March and early April in Parque Nacional Braulio Carrillo. Testes measurements 

 of two adult males were 10x6 mm and 8x6 mm. A single nymph of Ixodes 

 venezuelensis was collected on M. mexicana and represents the first record of this 

 tick on the Mexican mouse-opossum. 



Specimens examined (14)— Finca La Selva, 3 km S of Puerto Viejo [1 female, 

 KU; 1 male, LACM]; 1 1 km S, 4.5 km W of Puerto Viejo, 270 m [1 male, UNA], 

 325 m [6 males, 2 females, FMNH; 1 male, UNA], and 340 m [1 sex?, FMNH]; 

 1 km S, 11.5 km E of San Miguel, 710 m [1 male, FMNH]. 



